Offshore drilling of oil and gas wells is being conducted in constantly increasing water depths. Such offshore drilling operations in active ocean and sea oil exploration areas are now being conducted in depths of 1,000 ft to 3,000 ft, and will be conducted at depths of 5,000-6,000 ft in the foreseeable future.
Offshore drilling in deep water is commonly conducted from a column stabilized semisubmersible drilling vessel such as described and discussed in U.S. Patent to Lloyd U.S. Pat. No. Re. 29,167 and in OCEAN INDUSTRY 1976-1977 Directory of Marine Drilling Rigs published Sept. 1976 by Gulf Publishing Co. of Houston Tex. Such drilling vessel is semisubmerged so that the mean waterline of the ocean surface is at approximately one-half the height of the columns and the vessel operates in column stabilized condition with its main deck above the ocean surface, thereby providing a semisubmerged floating platform which supports the drilling rig or derrick used for conducting drilling operations.
Typically the drilling rig or derrick has a main drilling floor disposed above the main deck of the vessel; and a drilling "rotary" located on the main drilling floor of the derrick is a key element in carrying out drilling operations. Such semisubmersible vessels are also provided with a "spider deck" located below the vessel's main deck at a vertical elevation whereby there generally is a distance of about 30 ft between the spider deck and the rotary on the drilling floor of the derrick. The spider deck also is located so that generally this deck is a suitable distance above mean water line of the ocean surface while the vessel is in column stabilized semisubmersible condition for conducting drilling operations.
In drilling a deep water oil or gas well, a "temporary guide base" is installed on the sea floor where the well is being drilled. Then a "conductor pipe", a well head housing, a well head connector, and permanent guide base are installed in cooperation with the temporary guide base, and the same are effectively secured to the ocean floor by cementing the conductor pipe to a hole made in the sea floor during an initial stage of drilling the well.
A blow out preventer stack (sometimes called "BOP" in the trade) is mounted on the permanent guide base in operative relationship with the well head connector to control the well during drilling operations, and particularly to prevent a "blowout" of the well during drilling and/or production.
In a typical oil well drilling operation, the blow out preventer stack (BOP) is lowered from the drilling vessel to the sea floor by means of a series of marine "risers". Typically a riser is 50 ft long and includes a main central relatively large pipe section (typically 185/8" O.D.) with a top and bottom section for connecting the riser to two other risers at each end, thereby forming a vertical series or string of risers ultimately extending between the sea bottom and the drilling vessel. Each riser typically also includes four small pipes extending axially of the riser from end to end and circumferentially spaced around the main central pipe section at approximately 90.degree. intervals. Two of these pipes are circumferentially spaced 180.degree. and serve as a kill line and/or choke line, respectively, for conducting and delivering mud or the like to the blow out preventer to appropriately control pressure in the well hole thereby preventing or stopping a blow out of the well. The other two pipes on each riser are generally hydraulic supply lines
Typically the blow out preventer stack has at its top a "lower riser package" and a ball joint or flex joint plus a riser adapter. These components make it possible to connect the lowermost riser to the blow out preventer (BOP) whereby a string of risers are connected to the blow out preventer and ultimately extend from the sea bottom to the drilling vessel in a manner so that the riser string may pivot and/or rotate around theoretical "Y axis" extending vertically upward from the well; such movement is required under typical drilling and/or production conditions, and generally up to 10.degree. departure of the riser string from the vertical Y axis is desirable.
For more detailed background information regarding suitable deep water drilling equipment and operations such as herein discussed, reference is made to a brochure published by Vetco Offshore, Inc. of Ventura, Calif. entitled "Deep Water Drilling Equipment" (hereafter called "Vetco Brochure") which contains the following table of contents.
Section I Introduction
a. Introduction to Guidelineless Drilling PA1 b. Operational Sequence Set PA1 a. Guidance Unit - Guidelineless PA1 a. Blowout Preventer Stack PA1 a. Lower Marine Riser Package PA1 b. Marine Riser Joints and Buoyancy Kits PA1 c. Upper Marine Riser Package
Section II Expendable Wellhead Equipment
Section III Blowout Preventer System
Section IV Marine Riser Systems
(Said Vetco Brochure is incorporated herein by reference as to background prior art details regarding pertinent apparatus and/or methods not specifically shown or discussed herein.)
In deep water drilling it is generally necessary to desirable to support the weight of the string of risers extending between the vessel and the blow out preventer, including when the B.O.P. is in place on the sea floor. One of the systems for doing this is to provide buoyancy modules attached to each riser, whereby surrounding sea water provides buoyancy forces which sufficiently vertically support the string of risers extending from the BOP on sea floor to the vessel at the surface.
Also, in a typical deep water drilling system, a pair of control cables extends from the vessel to the BOP to control and operate various components of the BOP system; such control cables may be designed for electrical or hydraulic control of the blow out preventer system. Each control cable generally extends from one of two winches mounted on the drilling vessel so that the control cable having its lower end connected to the BOP can be raised or lowered by each respective winch as necessary or desirable. Generally such control cable winches are mounted on a column stabilized semisubmersible drilling vessel below the main deck and above the spider deck.
When drilling in deep water exceeding 1,000 ft, and especially in depths of 3,000-6,000 ft, the elongated control cables for the BOP have a substantial weight. It therefore has been found necessary or desirable to supplement and reduce the load imposed on the control cable winches (and on upper portions of the control cables) by clamping each control cable to the above discussed risers, thereby utilizing the buoyancy force provided by the buoyancy modules on the risers to support at least partially the weight of such control cables. This also makes it possible to reduce winch capacity which is generally desirable.
Besides using such risers and the resultant riser string to lower the blow out preventer (BOP) to the sea floor as above discussed, the riser string and control cables supported by same establish an operational arrangement for conducting drilling operations and controlling the well from the vessel, in a manner known to those skilled in the art (see for example aforementioned Vetco Brochure). Additionally, for maintaining the operational drilling system it is sometimes necessary or desirable to lift the risers back up onto the drilling vessel together with the control cables; as, for example, to retrieve the lower riser package and an associated upper portion of the BOP stack for purposes of servicing, maintenance, replacement or the like.
When the B.O.P. stack is being lowered with the risers as above discussed, a typical operational sequence includes the following: The blow out preventer stack (BOP) is mounted on the spider deck of the vessel below the main deck. A first riser is lowered through the rotary on the drill floor of the derrick on the vessel, and the lower end of that riser is connected to the BOP riser adapter by men working on the spider deck. Sometimes this first and lowest riser is not of standard riser length (e.g. 50 ft) to adjust for the distance between the sea bottom and the mean waterline, including an adjustment for tide (such a lower-most non-standard riser is sometimes called a "pup joint"). At the drilling floor of the derrick on the vessel, the lower portion of another riser is connected to the upper portion of the previously installed riser; and the thus-connected risers are lowered through the rotary. A string of risers are successively built-up in this way and lowered with the BOP stack, in a manner known in the art. Two control cables are also fed from the aforementioned winches located below the main deck and clamped to the kill and choke lines respectively by two men working on the spider deck, whereby such control cables are lowered with the riser string and BOP stack. This is accomplished by each man clamping one control cable to a kill line or choke line of a riser by means of a cable clamp made and used according to the prior art or made and used according to this invention, as further discussed below. In raising the risers to lift the riser string and upper part of the blow out preventer onto the vessel, as for maintenance or the like, the above discussed operations are reversed. Particularly, the risers are sequentially raised through the rotary on the drill floor of the derrick on the vessel and there disconnected; and the men on the spider deck sequentially remove the cable clamps from each riser section so that the control cables may be reeled on the respective winches.
It is necessary that each riser can be raised and lowered through the standard rotary on the drill floor of the derrick without damaging any part of the risers and/or rotary, and without otherwise hindering operations at the rotary. The rotary has an opening of fixed maximum diameter, which generally is 491/2 inches. Practical considerations for efficient operations require suitable minimum clearance between the rotary opening periphery and all parts of all risers, e.g. to avoid problems due to roll and/or pitch motion of the drilling vessel relative to the riser string. Therefore, trade practice requires that all risers and components thereof, and all items mounted on same, must be disposed within a maximum diameter of 40 inches. That is, it is commercially unacceptable to have any component of a riser, or anything mounted on a riser, extend radially outward from the central axis of the riser more than 20 inches. This limitation is particularly applicable to any control cable clamps or portions thereof mounted on the riser kill line and/or choke line (or any other parts of the risers) for supporting and/or raising and/or lowering the control cables for the BOP, as above discussed.
The most pertinent prior control cable clamp arrangement presently known to applicant is a control cable clamp made by or for Regan Offshore International, Inc. of Torrance, Calif. and supplied by that company to applicant's assignee, Santa Fe International Corporation of Orange, Calif., for use on a drilling vessel "Pacnorse" operated in a joint venture in the North Sea by Santa Fe Drilling Company which is a division of Santa Fe International Corporation (such clamp is sometimes hereafter called the "prior art control cable clamp").
Such prior art control cable clamps involve a number of serious problems and shortcomings from the view point of different construction, mode of operation and results, and also involves materially different concepts, although directed to solving like problems as the present invention of this application.
One substantial problem encountered with such prior art control cable clamps is that they require two-step operations for mounting a clamp subassembly on the riser's kill line and choke line and for separately clamping each cable to such clamp subassembly; and a similar two-step operation in reverse is required for disengagement of the clamp subassembly from the riser and unclamping the control cable to reel same on its winch. Alternatively, the control cable must first be clamped or unclamped with respect to the movable clamp subassembly, which movable subassembly must then be installed on or removed from the clamp portion fixed on the riser. These relatively complex multi-step time consuming clamping and unclamping operations for reeling out or reeling in the control cables are carried out by two men stationed on the spider deck which is a relatively exposed position and can be hazardous if the seas are moderately heavy or greater. Also it is necessary for the two men on the spider deck to manually remove from the risers a rather large and cumbersome clamp subassembly in order that all remaining components on the riser are within the above discussed 20 inch radial maximum distance from the central axis of the riser to assure unhampered raising and/or lowering of the risers through the rotary on the drill deck as above discussed. Such multi-step operations which are quite cumbersome and time consuming thereby slow the overall operation of the drill rig and drilling vessel, thereby adversely penalizing the overall well drilling system which has a very high cost of operation per unit time. Furthermore, such prior art control cable clamps contain clamping components which are not as sturdy or reliable as desired (and achieved according to the present invention). Still further, such prior art control cable clamps are not readily and suitably adjustable to accommodate for variations in manufacture and/or due to wear and tear from using the cable clamps and/or due to variation in the control cables because of manufacturing tolerances and/or usage. Other disadvantages of such prior art control cable clamps and the resultant substantial problems and adverse penalties imposed by the same on the overall offshore drilling system and its operations in very deep water will be apparent to those skilled in the art in light of the explanation herein with reference to the present invention disclosed and claimed in this application.
It is an object of this invention to provide a new improved control cable clamp and riser system, and components thereof, and a new improved system and method of using same, which avoid above discussed and other shortcomings of the best known prior art and provide many significant improvements and advantages by virtue of utilizing new and different concepts, constructions, modes of operation and results, as shown below and as will be apparent to those skilled in the art in light of the disclosure of this application.